Drag conveyors of the type disclosed herein receive particulate material such as grains, pet food, plastics, animal feed, powders, flour, coffee beans and some paste-like substances etc. at an inlet to the machine and transfer them to a second location where they may be blended, bagged, mixed, stored, pelletized, etc. Various types of drag conveyors have been in use for almost a century. With the exception of the design disclosed in pending U.S. patent application Ser. No. 08/905,012 filed on Aug. 11, 1997 by Jon F. Baker, now U.S. Pat. No. 5,947,251, issued Sep. 7, 1999, only one type of drag conveyor design is believed to have the true capability of being self-cleaning to some extent, i.e., capable of removing all material from the conveyor at the completion of a run. This latter conveyor has a U-shaped trough, and can perform in a self-cleaning capacity so long as the paddles that drag the material through the trough maintain the shape of their edges. Paddles for such conveyors are typically made of a conventional ultra-high molecular weight plastic to minimize friction against the typical metal sheet steel trough, but even that material is subject to wear after time. An example of a U-trough conveyor is shown in U.S. Pat. No. 5,174,433 granted to Robert H. Moser and assigned to InterSystems, Inc. of Omaha, Nebraska on May 21, 1992. U.S. Pat. No. 5,947,261 incorporated herein by reference for its disclosure of the preferred type of drag conveyor with which my improvements are useful. To the extent that a round-bottomed or U-trough drag conveyor is referred to herein from time to time, the Moser patent is incorporated herein as exemplary of such a design.
Whenever the Baker, Moser or other conveyors are required to move the material from a first level to a second higher level, they typically receive the material at an inlet hopper of a first horizontal section, transport it to an upwardly-curved section known in this particular art as a "bend section", and discharge the material either from a straight upwardly-inclined section or from a straight "header end" at which the material is immediately discharged just beyond the bend section.
Upward inclined conveyance of granular material requires that a divider pan be utilized between the runs of a continuous chain through the lower, carrying run of conveying paddles and an upper, return run of the return paddles. The pan typically extends at least from the inlet end of the bend section all the way through to the end of the drag conveyor. This divider pan inhibits material from spilling over or flowing backwardly over the tops of the carrying paddles opposite to the carrying direction due to gravity, as they would tend to do in the absence of a divider pan. The Moser type of round-bottomed conveyor attempts to solve this problem by confining or entrapping the carrying paddles against the bottom of the trough, through use of the divider pan as a hold-down bearing surface contacting the top, horizontal edges of the paddles. The load-receiving pan thus serves to maintain tension in the chain as the paddles pass around the inside curve of the bend section. This confining of the round-bottomed paddles requires that the bend section and the divider pan both be designed about a large radius. Obviously, since the trough of a round-bottomed bend section is curved and passes about a radius spaced some ten feet or more above the bend section, the compound curve and round or U-shaped bottom present design and manufacturing problems of some significance. Further, most but not all drag conveyors for particulate material require that the upper horizontal edges of the paddles be beveled to prevent the collection of what is commonly referred to as "soldiers", particulate material standing on the upper edges of the paddles in the lower or carrying portion of the chain run. If the same material was always being conveyed, soldiers are not a problem. But where converting from one kind of material to another, the existence of soldiers can present a significant problem. If converting from the transporting of dog food to cat food or vice versa, for example, no purchaser of dog food wants cat food in the purchased bag. The same is true for the purchaser of cat food. Carryover of material from one run to the next is undesirable in most cases, and totally unacceptable in others. To avoid collection of soldiers, the tops of the paddles are preferably and sometimes necessarily beveled downwardly and forwardly, the latter so that soldiers fall in front of the paddle on which they would tend to collect, but are prevented from doing so by the bevel. These beveled edges typically come to a near point or sharp edge. That means that if a Moser-type conveyor used beveled edge paddles, utilizing the divider pan as a bearing surface to hold the round paddle bottoms against the trough bottom during passage through the bend section, would wear against the sharp bevel edges of the paddles while these edges are in contact with the divider pan. That load can be heavy, since the divider pan is the sole chain tension provider in the carrying run of the bend section. Because of this, a Moser-type conveyor often sacrifices the benefit of beveled edge paddles in favor minimizing paddle wear, thus making the unit less effective for cleaning itself at the end of a run for certain materials. It's a trade-off between either slowing down paddle wear or eliminating the soldier problem through use of beveled paddles. In the upper return run of the chain, tension may be maintained in the bend section by the rounded bottoms of the paddles riding against a specially-constructed inverted round bottom or against a pair of wear rails extending about a curve corresponding to the bend section curvature. At least in the upper run, the wider edges of the paddles would provide the bearing surface, rather than the sharper beveled edges of the tops of the paddles taking the brunt of the load, as in the carrying run, were paddles with bevels to be used.
Square paddles are also used in drag conveyors, but are typically used in situations where self-cleaning capability is not a prerequisite. Such square paddles require some small measure of side edge clearance to avoid skewing as they travel along the trough. When used in a bend section, although it is not known precisely how they confine the carrying paddles, presumably they too would utilize the divider pan or perhaps guide rails to hold the paddles down against the trough bottom. Also presumably, the hold-down means would contact the upper beveled edges of the paddles, if in fact they are beveled, causing wear to ensue more rapidly than if wider, non-beveled edges were made to accept the brunt of the bend section wear. My understanding is that square paddles do not have the benefit of beveled edge paddles, and such conveyors find more frequent use in environments where self-cleaning is not a prerequisite.
Many conveyors other that those described in the herein-incorporated Baker '261 patent and Moser '433 reference necessitate the use of idler sprockets for supporting the upper return run of the continuous chain. As recognized in the Baker reference, these idler sprockets are a significant maintenance problem requiring excessive down-time of the conveyor and of machinery fed by the conveyor whenever sprocket bearings must be changed. A need exists for a drag conveyor having a bend section, in which the conveyor trough sides provide the sole support for beveled edge paddles throughout the entire carrying and return runs, and do so without requiring the use of the beveled edges as load-bearing components.